6R-BH4 is known to be a common coenzyme for tyrosine hydroxylase and tryptophan hydroxylase which are rate-limiting enzymes in the biosynthesis of neurotransmitters such as dopamine and serotonin. It is considered that 6R-BH4 is a regulating factor in the biosynthesis of these neurotransmitter amines since 6R-BH4 is contained in nerve endings only in an amount which is approximately the Km value of each hydroxylase. In fact, a shortage or decrease in enzymes which participate in the biosynthesis of this coenzyme from GTP will give rise to a decrease in neurotransmitter amines, thus resulting in various neuropsychiacric diseases. Actually, malignant phenylketonuria was discovered in 1974 and subsequent studies have revealed that autopsied brain specimens and cerebrospinal fluid of patients with Parkinson's disease and Alzheimer's dementia show a decrease In 6R-BH4 content. It has also been found that supplementation therapy by 6R-BH4 is effective in the treatment of malignant phenylketonurla, juvenile Parkinson's disease and, in accordance with recent studies, infantile autism of the dysbolism type. Based on these facts, over a hundred 6R-BH4 derivatives have been drug-designed in Switzerland, the United States and Japan and screened for their coenzyme action analogous to 6R-BH4. However, there has not been discovered any derivative which is superior to 6R-BH4 in terms of its action. In addition, no finding has been made hitherto relating to the biochemical cellular responses of 6R-BH4.